How can I get a volume to expand uniformily in all directions, like a nova?
The smoke by default rises upwards from the emitter. To disable this, you need to set the Density and Temp. Diff. both to 0. Also disable the influence of gravity by setting Gravity under Smoke Field Weights to 0.



Here, I've also set Dissolve to 10 frames, to make the smoke disappear. I'm using a Force type force field, with Power set to 2*. This makes the effect falloff physically correct, by making the force proportional to inverse square of the distance from the source.



And this is the result I got.



As you can see, it doesn't spread exactly equally in all directions. However, the force, and most importantly the Vorticity of the smoke domain, are quite strong. The smoke will be calmer if you reduce the vorticity, and spread more slowly if you reduce the strength of the force.

*Have a look at the Blender Manual for an explanation of the Power and other settings.